Hitherto, serious accidents have occurred in workpiece shape-altering machines, such as stamping presses, when the workpiece has been improperly positioned in the die-set thereof or when the stamped workpiece sticks in the upper die or punch and is carried upward while the new workpiece has been automatically fed into the die-set by the conveyor machine. The result has been that when the punch or upper die has again descended, the workpiece adhering to it has clashed with the next workpiece, which has meanwhile been fed to the die set, with dangerous consequences to the operator as well as to the machine. Previously, workpiece-positioning systems have been provided with limit switches in conjunction with an electrical circuit for attemptedly indicating when a workpiece blank has been improperly positioned in the press die-set, but such limit switches have often failed as a result of their repeated operations, with the result that the intendedly safeguarding system no longer operates successfully. In one instance, for example, the inspection system did not indicate the improperly positioned workpiece, with the result that the workpiece was hurled across the room by the improper contact of the punch therewith, causing serious danger to bystanders.